rproxy test: send the body over its limit without Connection: close (closes #371)

visit's client sends Connection: close, and Go's server then closes the
connection after the 413 without reading the rest of the body. The
visitor, still sending, could see the reset before the answer. The test
now sends the request on its own connection without that header, so Go's
server reads the rest before it closes.

Model: opus-5-5

internal/rproxy/limits_test.go

@@ -93,10 +93,15 @@ func TestABodyAnnouncedOverItsLimitIsRefusedOnceWAFAllowsIt(t *testing.T) {
 				},
 			)
 
-			got := visit(t, newRequest(t, http.MethodPost, addr, "/",
-				bytes.NewReader(make([]byte, tc.size))))
-			if got.status != http.StatusRequestEntityTooLarge {
-				t.Errorf("answered %+v, want 413", got)
+			// The visitor sends no Connection: close, so Go's server reads the rest
+			// of the body before it closes the connection. Closed with the rest
+			// unread, the connection is reset while the visitor still sends.
+			conn := dial(t, addr)
+			send(t, conn, post+announcedLength(tc.size)+
+				strings.Repeat("x", tc.size))
+
+			if status := readStatus(t, conn); status != http.StatusRequestEntityTooLarge {
+				t.Errorf("status %d, want 413", status)
 			}
 
 			if asked := standIn.question(t); len(asked.body) != tc.asked {